Technically, in Germany, unions always negotiate only for their members. And the employers can freely choose with whom and if at all they negotiate.

Now de facto unions tend to have a unionization grade of 30% or higher in the firm or the sector and so can force the employers to negotiate. The employers then pay everybody according to the treaty, sometimes because the individual contracts are linked to collective contracts, sometimes because paying non union members less would be an encitement to join the union.

But especially in sectors or firms were unions are weak, small, almost virtual unions can make treaties too. In the individual treaties where is then a clause: pay according to collective treaty with christian union x. And you can claim to have a collective treaty. And you could use such a treaty with a pliant union to ward off a general minimum wage.

Or "union" and employers could even try to make the declaratory treaty mandatory in their sector. Mandatory treaties mostly exist in sectors were unions and employers associations are weak anyway.

Now anywhere were real unions exist, they could just go on strike. But in the many, especially service, sectors were real unions are weak, they are not able to go on strike.

There is one remedy: The courts. (It is Germany after all). Real unions can go to court an declare their unions non-union. If they are to weak to plausibly conduct collective bargaining, they should be declared non-unions, always in a particular sector. The unions succeeded with such a suit in the lend-labour sector recently.

But that needs years and even if the real union succeeds, in the meantime a new fake union has popped up.